It is well known to provide a low noise amplifier (LNA) at an input of a high frequency, e.g. microwave, receiver in order to amplify small signals which are desired to be received. It is also well known to provide, at an input to the LNA, a microwave power limiter which serves to limit the power of signals and interference which may be supplied to the LNA, in order to protect the LNA from damage which may otherwise be caused by an excessive input power.
For example, in a radar system an output of a transmit high power amplifier (HPA) may be coupled via a circulator to an antenna, and the antenna may be coupled via the circulator and a microwave power limiter to the input of the receiver LNA. The microwave power limiter serves to protect the LNA from excessive input power, either coupled from the HPA (for example by reflection from the antenna) or received via the antenna from another transmitter.
Typically, the microwave power limiter comprises a PIN (P-type, intrinsic, N-type semiconductor) diode, or a plurality of PIN diodes coupled via a transmission line. At microwave frequencies, the recombination time of carriers in the intrinsic semiconductor region of a PIN diode is much greater than the microwave signal cycle time, so that the PIN diode has a slow turn-off time and presents a low resistance to a microwave signal having excessive power.
Consequently, such a microwave power limiter can be very effective, but it involves disadvantages of a high cost and significant space, being assembled from discrete components typically using an epitaxial silicon semiconductor process, and loss can be introduced at the interface between the limiter and the LNA.
The LNA is typically implemented using a GaAs (Gallium Arsenide) MESFET (metal-semiconductor field effect transistor) or GaAs HEMT (high electron mobility transistor) integrated circuit. It would be desirable to integrate the microwave power limiter with the LNA, but this is not practical with epitaxial silicon PIN diodes.
A process-compatible planar GaAs PIN diode has been proposed using selective implantation of P-type and N-type regions in a GaAs semiconductor material. However, this provides a lateral arrangement (rather than a vertical arrangement as in the epitaxial silicon PIN diode) which has the disadvantages of requiring close spacing of the implanted regions, and hence a high resolution, and leakage of carriers from the intrinsic semiconductor between the implanted regions into the remainder of the substrate. Further, the implanted P-type region in this case uses zinc, which creates other problems for combined fabrication processes for MESFETs and HBTs (heterojunction bipolar transistors).
It is also known that Schottky diodes are intrinsic to the MESFET and HEMT processes. A Schottky diode can comprise a metal electrode over an N− region in a GaAs substrate, a depletion region being formed beneath the electrode. A Schottky diode provides a low forward voltage and low resistance, so that it can pass a high current, and has a fast response because there is little charge storage. The fast response of the Schottky diode makes it unsuitable as a replacement for a PIN diode in a microwave power limiter.
It is also known to use diodes, which may be Schottky diodes, for voltage clipping at an input to an FET circuit. For example, an input terminal may be coupled to an input of the FET circuit via a series resistance, and a diode may be connected between the input of the FET circuit and each of its supply voltage rails, to limit voltage swing at the input of the FET circuit to the supply voltages plus the forward voltage drops of the diodes.
Such an arrangement is unsuitable for use at high frequencies, for example of 1 GHz or more, because the capacitance of the diodes in combination with the series resistance introduces a signal distortion and a loss which are not acceptable (as well as a delay which may be acceptable). In addition, such an arrangement constitutes a voltage limiter or clipper, rather than a power limiter.
Thus there is a need to provide an improved power limiter for high frequency, e.g. microwave, signals.